This invention is directed to the art of making silicon nitride from silicon powder as a starting material. The powder is subjected to a reactive nitriding gas to form a mixed phase (alpha and beta) silicon nitride body, the body being then hot pressed. Within this art there are known two ways to agglomerate the powder materials: (a) by hot pressing as the last stage in the process, and (b) by cold compacting as an early stage in the process.
The prior art predominantly employs the first way. Agglomerating by hot pressing involves nitriding a loose supply of uncompacted silicon powder to form a cake of unconsolidated silicon nitride. The cake is broken up by primary and secondary reduction techniques to form a free powder supply of silicon nitride. The powder is then poured into a hot pressing assembly where it is agglomerated by heat and pressure. Unfortunately, the reduction techniques applied to the cake introduce contaminants to the silicon nitride powder. Such contaminants can form flaws within the final product. In addition, this powder technology often transfers an unreacted portion of powder ingredients directly into the hot pressing steps, requiring high hot pressing temperatures to bring about sintering under pressure.
The second way uses cold compacting as a step prior to the nitriding treatment. This results in a minimum green density for the material prior to nitriding of at least 1.1 gm/cm.sup.3, and particularly 1.1-1.7 gm/cm.sup.3 (which is approximately 53-85% of full silicon nitride theoretical). This is particularly illustrated in U.S. Pat. No. 3,839,540. The consolidation of the powder, by compacting, adversely affects nitriding by reducing the available pore network and associated exposed surface area. This must be undesirably countered by a greatly increased nitriding time to reduce incomplete nitriding. This in turn adversely affects the temperature of reaction within the material and the type of crystallographic structure formed as a result of nitriding. Without the right crystallographic structure, hot pressing requirements are aggravated if full density is to be achieved.
What is needed is a process which increases the gas phase formation of silicon nitride, dramatically cuts the amount of time required to carry out effective nitriding of a silicon mixture with its accompanying additives, reduces substantially the temperature requirements for hot pressing, and substantially reduces the possibility for flaws in the resulting hot pressed product.